1. Field of Invention
This invention relates generally to information storage systems which rely on the optical properties of a medium for controllable storage of information and, more particularly, to the actuator which directs a radiation beam toward, and/or detects a radiation beam which has interacted with, the storage medium.
2. Description of the Prior Art
Optical storage systems, especially those systems in which the information is stored on an optical disk-type medium, typically have read/write heads or actuators which provide the mechanism by which the radiation is applied to and received from the storage medium. The radiation beam interacts with localized regions of the information storage medium and, as a result of that interaction, the properties of the radiation beam transmitted through the optically encoded medium or reflected from the optically encoded medium are altered. By proper processing, the information stored on the medium can be recovered from the resulting radiation beam.
As higher densities of stored information in the storage media have been achieved, the requirements on the radiation beam have become more stringent. The higher densities of information on the media require that both the tracking of the applied radiation beam (to follow a path having sequential information patterns) and the focus of the applied radiation beam (on the individual information regions) be controlled within narrow limits to maximize the signal to noise ratio. By way of specific example, a portion of the radiation beam which has interacted with the storage medium can be tested by using radiation detector(s) with appropriate optical stops to determine whether radiation beam meets certain shape and size constraints. When the dimensional constraints are violated, the radiation beam is considered out of focus and the signals from the radiation detectors are used to control position apparatus. The positioning apparatus adjusts the position of an objective lens used in focussing the applied radiation. The positioning adjustment results in the focussing of the radiation on the optical storage medium. As will be clear, an externally applied force or acceleration of the storage system can result in a change of the position of the objective lens relative to the storage medium, i.e., because of the inertial force acting on the mass associated with the objective lens. The change in position results in the loss of information until the feedback loop associated with the focussing control can reestablish the appropriate position of the objective lens relative to the storage medium. Heretofore, the applications of the optical storage systems have been selected such that external forces and accelerations are small or infrequent during operation of the optical information storage system. However, applications have been proposed for the optical information storage systems which can involve the possibility of unpredictable external forces and accelerations.
In the past, attempts have been made to minimize the impact of selected external forces and accelerations on the actuator associated with an optical storage system. U.S. Pat. No. 4,796,245, issued Jan. 3, 1989 in the name of van Alem et al, discloses an example of a two-axis rotary actuator. The mass distribution of the lens holder is balanced so that lateral accelerations will not disturb the tracking orientation. No attempt is made in the reference to protect the (vertical) focus motion from disturbances. U.S. Pat. No. 4,403,316, issued Sep. 6, 1983 in the name of van de Veerdonk, is similar to the van Alem reference, but is applied to a rotary radial-access device rather than a fine-tracking/focus actuator. U.S. Pat. No. 4,516,231, issued on May 7, 1985 in the name of Michaelis, provides a technique for balancing motions induced by motor forces. Vibration and reaction forces from the motor are attenuated before interacting with the drive mechanism. External perturbations to the system are not addressed by this reference. U.S. Pat. No. 4,813,033, issued on Mar. 14, 1989 in the name of Baasch et al, describes an actuator in which the motor forces are applied around the center mass of the moving part. However, the mass distribution of the moving part is asymmetrical in both the tracking and the focussing directions resulting in an adverse reaction to external forces.
The related art references do not address the problem of protecting the focussing apparatus against external forces and accelerations which would break the `lock` of the focus control feedback loop. High density storage systems for more dynamic applications require an actuator that is less susceptible to external disturbances.
It is therefore an object of the invention to provide an improved actuator for a optical information storage system.
It is a feature of the invention to reduce the sensitivity of an actuator of an optical storage system to impact and acceleration.
It is a more particular feature of the present invention to reduce the sensitivity of the focus control of an actuator for an optical storage system to impact and acceleration.
It is another particular feature of the present invention to provide a counter-weight to an actuator arm to compensate for impact and acceleration on the focus control of an actuator arm.